The genetic relatedness among several human and animal rotavirus strains has been studied by hybridization techniques in which genomic RNA of a virus is hybridized to single-stranded RNA labeled probes synthesized by in vitro transcription from other strains. Gel electrophoresis of the resulting hybrids and analysis of their nuclease resistance have given an indication of the degrees of homology between the strains studied. An initial series, in which 9 human rotavirus strains from Venezuela were examined, has now been followed by a study in which more than a hundred well characterized human rotavirus strains obtained throughout a complete year (1981) have been analyzed. The results obtained with both groups suggest the existence of two rotavirus "families." In hybridization studies with two serotypically distinct human rotaviruses, DS-1 and Wa, the majority of the strains (79 percent) hybridized significantly to the Wa probe but not the DS-1 probe. These hybridization findings have now been confirmed and complemented by quantitative analysis that further establishes the distinctness between the two prototype viruses. In such studies, it was found that only 17 percent of sequence homology exists between Wa and DS-1 viruses. In addition, that there was 60-100 pecent homology between many of the isolates studied and one of the prototype viruses.